For example, the device described in Patent Document 1 is known as a mounting unit to be mounted on an electromagnetic contactor. This device has a pair of electromagnetic contactors having an excitation coil, a normally closed contact and a normally open contact, and also terminals thereof and are disposed in parallel to one another and an interlock block serving as a reversible unit that has connection terminals for connecting to the terminals of the aforementioned electromagnetic contactors and incorporates a wiring portion that performs interlock wiring between the connection terminals.    Patent Document 1: Japanese Patent Application No. 563-187523.
A mounting unit to be mounted on the electromagnetic contactor, in addition to the aforementioned reversible unit, is also known as an auxiliary contact unit that is provided with auxiliary circuit terminals as auxiliary terminals of the main circuit terminals of an electromagnetic contactor.
However, the electromagnetic contactor described in Patent Document 1 mentioned hereinabove is a device designed specifically for mounting only the reversible unit and is not supposed to be used for mounting other mounting units such as the auxiliary contact unit via the reversible unit.
In recent years, configurations in which the user himself stacks and mounts, as desired, a plurality of mounting units on an electromagnetic contactor have been considered. For example, a configuration has been considered in which a pair of electromagnetic contactors is arranged side by side, a reversible unit is head-on mounted between the two electromagnetic contactors, and an auxiliary contact unit is then mounted on the reversible unit.
However, since there are mountable auxiliary contact units and non-mountable auxiliary contact units depending on the configuration of the electromagnetic contactor, an erroneous mounting preventing mechanism is provided that allows the mountable auxiliary contact units to be mounted on the electromagnetic contactor and prevents the erroneous mounting of the non-mountable auxiliary contact units.
The erroneous mounting preventing mechanism is usually configured by forming a contactor-side engagement portion engaging with a unit-side engagement portion formed on the mountable unit on a unit mounting portion mounting the mounting unit of the electromagnetic contactor. The erroneous mounting preventing mechanism allows a mountable unit to be mounted on the unit mounting portion and prevents a non-mountable unit, which cannot be mounted, from being mounted on the unit mounting portion.
Where an electromagnetic contactor is provided with the above-described erroneous mounting preventing mechanism, when the mountable unit is mounted on the unit mounting portion of the electromagnetic contactor, the mounting can be performed by contacting the mounting surfaces with one another, but when the non-mountable unit is mounted on the unit mounting portion of the electromagnetic contactor, mutual contact between the mounting portions is prevented and the erroneous mounting state can be easily determined.
However, the following unresolved problem is associated with the reversible unit to be mounted between the electromagnetic contactor and the auxiliary contact unit. Thus, the reversible unit is configured to be mountable on any electromagnetic contactor, regardless of the configuration of the electromagnetic contactor, and when the auxiliary contact unit is mounted on the reversible unit, individual engagement portions mating with the case-side engagement portion of the electromagnetic contactor and the engagement portion of the auxiliary contact unit should be formed on the reversible unit, and reversible units of at least two types, namely, a reversible unit mountable on the electromagnetic contactor and a reversible unit non-mountable on the electromagnetic contactor, should be formed.